Process for separating ketoses from alkaline-or pyridine-catalyzed isomerization products

ABSTRACT

A process for the liquid phase adsorptive separation of psicose from an aqueous feed mixture of monosaccharides containing psicose along with other aldoses and ketoses. The feed is contacted with a calcium-Y type zeolite in two stages. Psicose and fructose are selectively adsorbed to the substantial exclusion of other aldoses and ketoses. In a first embodiment, the extract from the first stage is psicose and fructose is eluted with the raffinate components. In a second embodiment, psicose is again recovered in high purity from the first place as a first extract material while a second extract material, comprising highly purified fructose, is also recovered. The process can be carried out on a commercial scale by means of a simulated moving bed flow scheme.

RELATED APPLICATION

This is a continuation-in-part of copending application Ser. No.811,588, filed December 20, 1985, now U.S. Pat. No. 4,692,514.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of art to which this invention pertains is the solid bedadsorptive separation of a ketose from other ketoses and aldoses. Morespecifically, the invention relates to a process for separating psicosefrom a mixture comprising psicose, fructose and one or more additionalketoses, and/or aldoses which process employs an adsorbent comprising acalcium-exchanged Y-type zeolite to selectively adsorb psicose from thefeed mixture.

2. Information Disclosure

The use of crystalline aluminosilicates in non-hydrocarbon separationsis known, e.g., to separate specific monosaccharides or classes ofmonosaccharides from carbohydrate feed mixtures. A specific example of aclass separation is given in U.S. Pat. No. 4,024,331 disclosing theseparation of ketoses from a mixture of ketoses and aldoses using a typeX zeolite. Specific monosaccharides such as glucose and fructose areisolated from a feed mixture containing the same by an adsorptiveseparation process using an X zeolite as taught in U.S. Pat. No.4,442,285.

This invention is particularly concerned with the separation of aketose, psicose, from another ketose, fructose, mixed with the aldoses,mannose and glucose. Heretofore, no feasible method for commerciallyseparating psicose from the other products of isomerization of aldosesugars, e.g., glucose, was available. Therefore, enzymatic means ratherthan simple isomerization methods have been used for the production offructose, because the enzymatic route minimizes the production ofpsicose. Now, with my discovery of a means for separating psicose fromfructose, and other ketoses and aldoses, the simpler, less costlyisomerization methods can be used to produce psicose-free fructose.There are two isomerization routes known for obtaining fructose fromglucose, namely by the reaction of weak alkali on glucose and thereaction of hot pyridine on glucose. Chemistry of the CarbohydratesPigman et al Academic Press Inc. NY, NY 1948, pages 41, 126-7.Similarly, isomerization of galactose by either of the isomerizationtechniques referred to above will produce a mixture of aldoses, taloseand galactose and a mixture of ketoses, sorbose and tagatose and may beseparated by means here disclosed. Furthermore, there is considerableinterest in the various L-sugars, which are believed to be low incalories and possibly non-metabolized, which cannot be madeenzymatically, but only by isomerization routes such as those mentionedabove. This invention applies to the L-sugars as well as D-sugars, andis seen to be an advantageous method for obtaining L-fructose, free ofcontamination by L-psicose, as well as purified L-psicose.

Data related to potential adsorbents for the separation of mannose fromother monosaccharides is set forth in U.S. Pat. No. 4,471,114. Thispatent contains data related to the use of a Y type faujasite exchangedwith calcium cations as an adsorbent for the separation of mannose fromglucose and other monosaccharides.

The separation of mannose from glucose is the subject of British Pat.No. 1,540,556. There the adsorbent is a cation exchange resin in saltform, preferably calcium form.

Neuzil et al U.S. Pat. No. 4,340,724 teaches the separation byadsorption of a ketose from an aldose with a Y zeolite exchanged withNH₄, Na, K, Ca, Sr, Ba and combinations at the ion exchangeable sites oran X zeolite exchanged with Ba, Na or Sr and combinations thereof.

The separation of psicose from other ketoses is stated in U.S. Pat. No.4,096,036 to be possible with ion exchange resins capable of complexingwith a polyol at a first temperature and dissociating the complex at asecond temperature in a thermal parametric pumping apparatus.

SUMMARY OF THE INVENTION

It is accordingly an objective of the present invention to provide aprocess for the separation of a ketose from a feed mixture which is thealkaline- or pyridine-catalyzed isomerization product of an aldohexoseor aldopentose using a Y type zeolite with calcium cations at cationexchanged sites.

The present invention is a process for separating a ketose from a feedmixture comprising the ketose and at least one other monosaccharideselected from the group consisting of ketopentoses and ketohexoses. Morespecifically, the process comprises contacting at adsorption conditionsa monosaccharide comprising a mixture of psicose, fructose and one ormore monosaccharides with an adsorbent comprising a type Y zeolitecontaining calcium cations at the exchangeable cationic sites,selectively adsorbing psicose preferentially to the fructose and to thesubstantial exclusion of the other monosaccharides, removing thenonadsorbed portion of the feed mixture from contact with the adsorbent,and thereafter recovering the psicose by desorption at desorptionconditions as a single extract containing only psicose or as twoextracts in which psicose and fructose are recovered as separateextracts. In the first case, fructose will be collected with the otherraffinate materials. In the latter case, fructose may be furtherprocessed to obtain highly purified fructose as a product in addition topsicose. Psicose can be further purified by contacting the psicoseextract with a second bed of Y-type zeolite exchanged by calcium at theexchangeable sites, selectively and preferentially adsorbing the psicoseand recovering the psicose in the extract by desorption at desorptionconditions with water as desorbent. This process is especially usefuland attractive for the separation and recovery of L-psicose and/orL-fructose from catalytic isomerization mixtures of L-sugars.

Other objectives and embodiments of the present invention relate tospecific feed mixtures, adsorbents, desorbent materials, operatingconditions and flow configurations, all of which are hereinafterdisclosed in the following discussion of the present invention.

DESCRIPTION OF THE INVENTION

At the outset the definitions of various terms used through thespecification will be useful in making clear the operation, objects andadvantages of this process.

A "feed mixture" is a mixture containing one or more extract componentsand one or more raffinate components to be separated by this process.The term "feed stream" indicates a stream of a feed mixture which passesto the adsorbent used in the process.

An "extract component" is a component that is more selectively adsorbedby the adsorbent while a "raffinate component" is a component that isless selectively adsorbed. The term "desorbent material" shall meangenerally a material capable of desorbing an extract component. The term"desorbent stream" or "desorbent input stream" indicates the streamthrough which desorbent material passes to the adsorbent. The term"raffinate stream" or "raffinate output stream" means a stream throughwhich a raffinate component is removed from the adsorbent. Thecomposition of the raffinate stream can vary. from essentially 100%desorbent material to essentially 100% raffinate components. The term"extract stream" or "extract output stream" shall mean a stream throughwhich an extract material which has been desorbed by a desorbentmaterial is removed from the adsorbent. The composition of the extractstream, likewise, can vary from essentially 100% desorbent material toessentially 100% extract components. At least a portion of the extractstream, and preferably at least a portion of the raffinate stream, fromthe separation process are passed to separation means, typicallyfractionators or evaporators, where at least a portion of desorbentmaterial is separated to produce an extract product and a raffinateproduct. The term "extract product" and "raffinate product" meanproducts produced by the process containing, respectively, an extractcomponent and a raffinate component in higher concentrations than thosefound in the extract stream and the raffinate stream.

The adsorbent materials of this invention comprise type Y crystallinealuminosilicates having calcium cations at cation exchange sites. Thetype Y crystalline aluminosilicates or zeolites can be furtherclassified as faujasites. As in the general case of all zeolites, thesecrystalline compounds are described as a threedimensional network offundamental structural units consisting of silicon-centered SiO₄ andaluminum-centered AlO₄ tetrahedra interconnected by a mutual sharing ofapical oxygen atoms. The space between the tetrahedra is occupied bywater molecules and subsequent dehydration or partial dehydrationresults in a crystal structure interlaced with channels of moleculardimension. Zeolites are more fully described and defined in U.S. Pat.Nos. 2,883,244 and 3,130,007 respectively, incorporated herein byreference thereto. The Y zeolites in the hydrated or partially hydratedform can be represented in terms of mole oxides as shown in Formula 1below, in which "M" is a cation having a valence up to 3, "n" is thevalence of "M", "w" is a value from 3 to 6 and "y" is a value up to 9,depending on the identity of "M" and the degree of hydration of thecrystal:

Formula 1

    (0.9±0.2 )M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O

The electrovalence of the tetrahedra is balanced by. the cation "M" ofthe above equation which occupies exchangeable cationic sites in thezeolite. These cations which after initial preparation are predominantlysodium may be replaced with other cations by ion exchange methods wellknown to those having ordinary. skill in the field of crystallinealuminosilicates. Such methods are generally performed by contacting thezeolite or an adsorbent material containing the zeolite with an aqueoussolution of the soluble salt of the cation or cations desired to beplaced upon the zeolite. After the desired degree of exchange takesplaces, the sieves are removed from the aqueous solution, washed anddried to a desired water content. By such methods, the sodium cationsand any nonsodium cations which might be occupying exchangeable sites asimpurities in a sodium-Y zeolite can be partially or essentiallycompletely replaced with other cations. It is essential that the zeoliteused in the process of my invention contains calcium cations atexchangeable cationic sites.

Typically, adsorbents used in separative processes contain zeolitecrystals and amorphous material. The zeolite will typically be presentin the adsorbent in amounts ranging from about 75 wt. % to about 98 wt.% based on volatile free composition. The remainder of the adsorbentwill generally be amorphous material such as silica, or silica-aluminamixtures or compounds, such as clays, which material is present inintimate mixture with the small particles of the zeolite material. Thisamorphous material may be an adjunct of the manufacturing process forzeolite (for example, intentionally incomplete purification of eitherzeolite during its manufacture), or it may be added to relatively purezeolite, but in either case its usual purpose is as a binder to aid informing or agglomerating the hard crystalline particles of the zeolite.Normally, the adsorbent will be in the form of particles such asextrudates, aggregates, tablets, macrospheres or granules having adesired particle size range. The adsorbent used in our process willpreferably have a particle size range of about 16-40 mesh (Standard U.S.Mesh). I have found that Y zeolite with calcium cations and amorphousbinders possess the selectivity and other necessary requirementspreviously discussed and is therefore suitable for use in my process.

Certain carbohydrates or so-called simple sugars are classified asmonosaccharides. These monosaccharides are hydroxyaldehydes orhydroxyketones containing one ketone or aldehyde unit per molecule andtwo or more alcohol functionalities. Thus monosaccharides are classifiedas aldoses or ketoses on the basis of their carbonyl unit. Ketoses andaldoses are further classified by their carbon skeleton length.Accordingly, five-carbon and six-carbon monosaccharides receive therespective names of pentoses and hexoses. Well-known aldohexoses includeglucose, mannose and galactose. Arabinose, ribose and xylose arewell-known aldopentoses. Examples of common ketohexoses are fructose,psicose and sorbose. Ribulose and xylulose are common ketopentoses. Thisinvention is a process for eparating psicose from feed mixturescontaining other ketopentoses and ketohexoses as referred to above.

Consequently, feed mixtures which can be utilized in the process of thisinvention will comprise a mixture of psicose and at least one otherketose. Potential feed mixtures can be found in isomerization productsof D-glucose and L-glucose. Such mixtures will usually containsignificant quantities of such monosaccharides as psicose, fructose,glucose, mannose and small quantities of polysaccharides, such as DP 4+,i.e. having a degree of polymerization of four and greater. The feedmixtures whether derived from natural sources or isomerization, willalso contain quantities of lesser known monosaccharides. A typical feedmixture for this invention will contain psicose, fructose, mannose,glucose and polysaccharides in respective proportions, based on weightpercent of solids, ranging from 0.5 to 90 wt. %. In addition, there maybe up to 10 wt. % solids of other lesser known sugars.

Although it is not clear what properties of the adsorbent areresponsible for the separation herein described, it appears that itcannot be attributed to pore size selectivity alone. Since psicose andfructose are separated from sugar molecules of similar size, it appearsthat steric factors as well as electrostatic attraction action play animportant role in the separation. While it is not possible toconclusively set forth the molecular interaction responsible for theadsorption, one possible explanation is a combination of cationattraction which varies the orientation of specific sugar molecules tothe pore opening on the adsorbent. This varied orientation can provide asuitable disposition of the particular structural configurationcorresponding to certain sugar molecules which coincides with the shapeof the adsorbent pore openings as altered by the presence of specificcations. Therefore, both electrostatic interaction as well as physicaland stoichiochemical considerations may provide the mechanism for thisseparation.

Although it is possible by the process of this invention to produce highpurity products, it will be appreciated that an extract component isnever completely adsorbed by the adsorbent, nor is a raffinate componentcompletely unadsorbed by the adsorbent. Therefore, small amounts of araffinate component can appear in the extract stream, and likewise,small amounts of an extract component can appear in the raffinatestream. The extract and raffinate stream then are further distinguishedfrom each other and from the feed mixture by the ratio of theconcentrations of an extract component and a specific raffinatecomponent, both appearing in the particular stream. For example, theratio of concentration of the more selectively adsorbed to theconcentration of less selectively adsorbed sugars will be highest in theextract stream, next highest in the feed mixture, and lowest in theraffinate stream. Likewise, the ratio of the less selectively adsorbedsugars to the more selectively adsorbed will be highest in the raffinatestream, next highest in the feed mixture, and the lowest in the extractstream.

Desorbent materials used in various prior art adsorptive separationprocesses vary depending upon such factors as the type of operationemployed. In the swing bed system, in which the selectively adsorbedfeed component is removed from the adsorbent by a purge stream,desorbent selection is not as critical. However, in adsorptiveseparation processes which are generally operated continuously atsubstantially constant pressures and temperatures to ensure liquidphase, the desorbent material must be judiciously selected to satisfymany criteria. First, the desorbent material should displace an extractcomponent from the adsorbent with reasonable mass flow rates withoutitself being so strongly adsorbed as to unduly prevent an extractcomponent from displacing the desorbent material in a followingadsorption cycle. Expressed in terms of the selectivity, it is preferredthat the adsorbent be more selective for all of the extract componentswith respect to a raffinate component than it is for the desorbentmaterial with respect to a raffinate component. Secondly, desorbentmaterial must be compatible with the particular adsorbent and theparticular feed mixture. More specifically, they must not reduce ordestroy the critical selectivity of the adsorbent for an extractcomponent with respect to a raffinate component. Additionally, desorbentmaterials should not chemically react with or cause a chemical reactionof either an extract component or a raffinate component. Both theextract stream and the raffinate stream are typically removed from theadsorbent in admixture with desorbent material and any chemical reactioninvolving a desorbent material and an extract component or a raffinatecomponent would reduce the purity of the extract product or theraffinate product or both. Since both the raffinate stream and theextract stream typically contain desorbent material, desorbent materialsshould additionally be substances which are easily separable from thefeed mixture that is passed into the process. Without a method ofseparating at least a portion of the desorbent material present in theextract stream and the raffinate stream, the concentration of an extractcomponent in the extract product and the concentration of a raffinatecomponent in the raffinate product would not be very high, nor would thedesorbent material be available for reuse in the process. It iscontemplated that at least a portion of the desorbent material will beseparated from the extract and the raffinate streams by distillation orevaporation, but othersseparation methods such as reverse osmosis mayalso be employed alone or in combination with distillation orevaporation. Since the raffinate and extract products are foodstuffsintended for human consumption, desorbent material should also benontoxic. Finally, desorbent materials should also be materials whichare readily available and therefore reasonable in cost. Suitabledesorbents for this separation comprise water and ethanol or mixturesthereof.

The prior art has recognized that certain characteristics of adsorbentsand desorbents are highly desirable, if not absolutely necessary, to thesuccessful operation of a selective adsorption process. Suchcharacteristics are equally important to this process. Among suchcharacteristics are: adsorptive capacity for some volume of an extractcomponent per volume of adsorbent; the selective adsorption of anextract component with respect to a raffinate component and thedesorbent material; and sufficiently fast rates of adsorption anddesorption of an extract component to and from the adsorbent.

Capacity of the adsorbent for adsorbing a specific volume of an extractcomponent is, of course, a necessity; without such capacity theadsorbent is useless for adsorptive separation. Furthermore, the higherthe adsorbent's capacity for an extract component the better is theadsorbent. Increased capacity of a particular adsorbent makes itpossible to reduce the amount of adsorbent needed to separate an extractcomponent of known concentration contained in a particular charge rateof feed mixture. A reduction in the amount of adsorbent required for aspecific adsorptive separation reduces the cost of a separation process.It is important that the good initial capacity of the adsorbent bemaintained during actual use in the separation process over someeconomically desirable life.

The second necessary adsorbent characteristic is the ability of theadsorbent to separate components of the feed; or, in other words, thatthe adsorbent possess adsorptive selectivity, for one component ascompared to another component. Relative selectivity can be expressed notonly for one feed component as compared to another, but can also beexpressed between any feed mixture component and the desorbent material.The selectivity, (B), is defined as the ratio of the two components ofthe adsorbed phase divided by the ratio of the same two components inthe unadsorbed phase at equilibrium conditions, as shown in Equation 1,below: ##EQU1## where C and D are two components of the feed representedin weight percent and the subscripts A and U represent the adsorbed andunadsorbed phases respectively. The equilibrium conditions aredetermined when the feed passing over a bed of adsorbent does not changecomposition after contacting the bed of adsorbent. In other words, thereis no net transfer of material occurring between the unadsorbed andadsorbed phases. Where selectivity of the adsorbent for two componentsapproaches 1.0, there is no preferential adsorption of one component bythe adsorbent with respect to the other; they are both adsorbed (ornonadsorbed) to about the same degree with respect to each other. As the(B) becomes less than or greater than 1.0, there is a preferentialadsorption by the adsorbent for one component with respect to the other.When comparing the selectivity by the adsorbent of one component C overcomponent D, a (B) larger than 1.0 indicates preferential adsorption ofcomponent C within the adsorbent. A (B) less than 1.0 would indicatethat component D is preferentially adsorbed leaving an unadsorbed phasericher in component C and an adsorbed phase richer in component D.Ideally, desorbent materials should have a selectivity equal to about 1or slightly less than 1 with respect to all extract components so thatall of the extract components can be desorbed as a class with reasonableflow rates of desorbent material and so that extract components candisplace desorbent material in a subsequent adsorption step. Whileseparation of an extract component from a raffinate component istheoretically possible whentthe selectivity of the adsorbent for theextract component with respect to the raffinate component is justslightly greater than 1.0, it is preferred that such selectivity bereasonably greater than 1.0. Like relative volatility, the higher theselectivity. the easier the separation is to perform. Higherselectivities permit a smaller amount of adsorbent to be used.

The third important characteristic is the rate of exchange of theextract component of the feed mixture material, or, in other words, therelative rate of desorption of the extract component. Thischaracteristic relates directly to the amount of desorbent material thatmust be employed in the process to recover the extract component fromthe adsorbent; faster rates of exchange reduce the amount of desorbentmaterial needed to remove the extract component and therefore permit areduction in the operating cost of the process. With faster rates ofexchange, less desorbent material has to be pumped through the processand separated from the extract stream for reuse in the process.

The adsorption-desorption operations may be carried out in a dense fixedbed which is alternatively contacted with a feed mixture and a desorbentmaterial in which case the process will be only semicontinuous. Inanother embodiment, generally referred to as a swing bed system, a setof two or more static beds of adsorbent may be employed with appropriatevalving so that a feed mixture can be passed through one or moreadsorbent beds of a set while a desorbent material can be passed throughone or more of the other beds in a set. The flow of a feed mixture and adesorbent material may be either up or down through an adsorbent in suchbeds. Any of the conventional apparatus employed in static bedfluid-solid contacting may be used.

Countercurrent moving bed or simulated moving bed flow systems, however,have a much greater separation efficiency than fixed bed systems and aretherefore preferred. In the moving bed or simulated moving bedprocesses, the retention and displacement operations are continuouslytaking place which allows both continuous production of an extract and araffinate stream and the continuous use of feed and displacement fluidstreams. One preferred embodiment of this process utilizes what is knownin the art as the simulated moving bed countercurrent flow system. Insuch a system, it is the progressive movement of multiple liquid accesspoints down a molecular sieve chamber that simulates the upward movementof adsorbent contained in the chamber.

Only four of the access lines are active at any one time: the feed inputstream, desorbent inlet stream, raffinate outlet stream, and extractoutlet stream access lines. Coincident with this simulated upwardmovement of the solid adsorbent is the movement of the liquid occupyingthe void volume of the packed bed of adsorbent. So that countercurrentcontact is maintained, a liquid flow down the adsorbent chamber may beprovided by a pump. As an active liquid access point moves through acycle, that is, from the top of the chamber to the bottom, the chambercirculation pump moves through different zones which require differentflow rates. A programmed flow controller may be provided to set andregulate these flow rates.

The active liquid access points effectively divide the adsorbent chamberinto separate zones, each of which has a different function. In thisembodiment of this process, it is generally necessary that threeseparate operational zones be present in order for the process to takeplace although in some instances an optional fourth zone may be used.

The adsorption zone, zone 1, is defined as the adsorbent located betweenthe feed inlet stream and the raffinate outlet stream. In this zone, thefeedstock contacts the adsorbent, an extract component is adsorbed, anda raffinate stream is withdrawn. Since the general flow through zone 1is from the feed stream which passes into the zone to the raffinatestream which passes out of the zone, the flow in this zone is consideredto be in a downstream direction when proceeding from the feed inlet tothe raffinate outlet streams.

Immediately upstream with respect to fluid flow in zone 1 is thepurification zone, zone 2. The purification zone is defined as theadsorbent between the extract outlet stream and the feed inlet stream.The basic operations taking place in zone 2 are the displacement fromthe non-selective void volume of the adsorbent of any raffinate materialcarried into zone 2 by the shifting of adsorbent into this zone and thedesorption of any raffinate material adsorbed within the selective porevolume of the adsorbent or adsorbed on the surfaces of the adsorbentparticles. Purification is achieved by passing a portion of extractstream material leaving zone 3 into zone 2 at zone 2's upstreamboundary, the extract outlet stream, to effect the displacement ofraffinate material. The flow of material in zone 2 is in a downstreamdirection from the extract outlet stream to the feed inlet stream.

Immediately upstream of zone 2 with respect to the fluid flowing in zone2 is the desorption zone, or zone 3. The desorption zone is defined asthe adsorbent between the desorbent inlet and the extract outletstreams. The function of the desorption zone is to allow a desorbentmaterial which passes into this zone to displace the contact with feedin zone 1 in a prior cycle of operation. The flow of fluid in zone 3 isessentially in the same direction as that of zones 1 and 2.

In some instances, an optional buffer zone, zone 4, may be utilized.This zone, defined as the adsorbent between the raffinate outlet streamand the desorbent inlet stream, if used, is located immediately upstreamwith respect to the fluid flow to zone 3. Zone 4 would be utilized toconserve the amount of desorbent utilized in the desorption step since aportion of the raffinate stream which is removed from zone 1 can bepassed into zone 4 to displace desorbent material present in that zoneout of that zone into the desorption zone. Zone 4 will contain enoughadsorbent so that raffinate material present in the raffinate streampassing out of zone 1 and into zone 4 can be prevented from passing intozone 3, thereby contaminating the extract stream removed from zone 3. Inthe instances in which the fourth operational zone is not utilized, theraffinate stream passing from zone 1 to zone 4 must be carefullymonitored in order that the flow directly from zone 1 to zone 3 can bestopped when there is an appreciable quantity of raffinate materialpresent in the raffinate stream passing from zone 1 into zone 3 so thatthe extract outlet stream is not contaminated.

A cyclic advancement of the input and output streams through the fixedbed of adsorbent can be accomplished by utilizing a manifold system inwhich the valves in the manifold are operated in a sequential manner toeffect the shifting of the input and output streams, thereby allowing aflow of fluid with respect to solid adsorbent in a countercurrentmanner. Another mode of operation which can effect the countercurrentflow of solid adsorbent with respect to fluid involves the use of arotating disc valve in which the input and output streams are connectedto the valve and the lines through which feed input, extract output,desorbent input and raffinate output streams pass are advanced in thesame direction through the adsorbent bed. Both the manifold arrangementand disc valve are known in the art. Specifically, rotary disc valveswhich can be utilized in this operation can be found in U.S. Pat. Nos3,040,777 and 3,422,848. Both of the aforementioned patents disclose arotary type connection valve in which the suitable advancement of thevarious input and output streams from fixed sources can be achievedwithout difficulty.

In many instances, one operational zone will contain a much largerquantity of adsorbent than some other operational zone. For instance, insome operations the buffer zone can contain a minor amount of adsorbentas compared to the adsorbent required for the adsorption andpurification zones. It can also be seen that in instances in whichdesorbent is used which can easily desorb extract material from theadsorbent, that a relatively small amount of adsorbent will be needed ina desorption zone as compared to the adsorbent needed in the buffer zoneor adsorption zone or purification zone or all of them. Since it is notrequired that the adsorbent be located in a single column, the use ofmultiple chambers or a series of columns is within the scope of theinvention.

It is not necessary that all of the input or output streams besimultaneously used, and in fact, in many instances some of the streamscan be shut off while others effect an input or output of material. Theapparatus which can be utilized to effect the process of this inventioncan also contain a series of individual beds connected by connectingconduits upon which are placed input or output taps to which the variousinput or output streams can be attached and alternatively andperiodically shifted to effect continuous operation. In some instances,the connecting conduits can be connected to transfer taps which duringthe normal operations do not function as a conduit through whichmaterial passes into or out of the process.

It is contemplated that at least a portion of the extract output streamwill pass into a separation means wherein at least a portion of thedesorbent material can be separated to produce an extract productcontaining a reduced concentration of desorbent material. Preferably,but not necessary to the operation of the process, at least a portion ofthe raffinate output stream will also be passed to a separation meanswherein at least a portion of the desorbent material can be separatedunder separating conditions to produce a desorbent stream which can bereused in the process and a raffinate product containing a reducedconcentration of desorbent material. The separation means will typicallybe a fractionation column or an evaporator, the design and operation ofeither being well-known to the separation art.

Reference can be made to D. B. Broughton U.S. Pat. No. 2,985,589, and toa paper entitled "Continuous Adsorptive Processing--A New SeparationTechnique" by D. B. Broughton presented at the 34th Annual Meeting ofthe Society of Chemical Engineers at Tokyo, Japan, on April 2, 1969(both of which are incorporated herein by reference), for furtherexplanation of the simulated moving bed countercurrent process flowscheme.

Although both liquid and vapor phase operations can be used in manyadsorptive separation processes, liquid-phase operation is required forthis process because of the lower temperature requirements and becauseof the higher yields of extract product that can be obtained withliquid-phase operation over those obtained with vapor phase operation.Adsorption conditions will include a temperature range of from about 20°C. to about 200° C. with about 20° C. to about 100° C. being preferredand a pressure range of from about atmospheric to about 500 psig withfrom about atmospheric to whatever pressure is required to ensure liquidphase being preferred. Desorption conditions will include the same rangeof temperatures and pressures as used for adsorption conditions.

The size of the units which can utilize the process of this inventioncan vary anywhere from those of pilot plant scale (see for example U.S.Pat. No. 3,706,812 to de Rosset et al) to those of commercial scale andcan range in flow rates from as little as a few cc an hour up to manythousands of gallons per hour.

Another embodiment of a simulated moving bed flow system suitable foruse in the process of the present invention is the cocurrent highefficiency. simulated moving bed process disclosed in U.S. Pat. No.4,402,832 to Gerhold, incorporated by reference herein in its entirety.

A dynamic testing apparatus may be employed to test various adsorbentswith a particular feed mixture and desorbent material to measure theadsorbent characteristics of adsorptive capacity, selectivity. andexchange rate. The apparatus consists of an adsorbent chamber ofapproximately 70 cc volume having inlet and outlet portions at oppositeends of the chamber. The chamber is contained within a temperaturecontrol means and, in addition, pressure control equipment is used tooperate the chamber at a constant predetermined pressure.Chromatographic analysis equipment can be attached to the outlet line ofthe chamber and used to detect qualitatively or determine quantitativelyone or more components in the effluent stream leaving the adsorbentchamber. A pulse test, performed using this apparatus and the followinggeneral procedure, is used to determine selectivities and other data forvarious adsorbent systems. The adsorbent is filled to equilibrium with aparticular desorbent material by passing the desorbent material throughthe adsorbent chamber. At a convenient time, a pulse of feed containingknown concentrations of a non-adsorbed polysaccharide tracermaltrin-DP₄₊, aldoses, and other trace sugars, all diluted in desorbent,is injected for a duration of several minutes. Desorbent flow isresumed, and the tracer and the aldoses are eluted as in a liquid-solidchromatographic operation. The effluent is collected in fractions andanalyzed using chromatographic equipment and traces of the envelopes ofcorresponding component peaks developed.

From information derived from the test, adsorbent performance can berated in terms of retention volume for an extract or a raffinatecomponent, selectivity for one component with respect to the other, andthe rate of desorption of an extract component by the desorbent. Theretention volume of an extract or a raffinate component may becharacterized by the distance between the center of the peak envelope ofan extract or a raffinate component and the peak envelope of the tracercomponent or some other known reference point. It is expressed in termsof the volume in cubic centimeters of desorbent pumped during this timeinterval represented by the distance between the peak envelopes.Selectivity, (B), for an extract component with respect to a raffinatecomponent may be characterized by the ratio of the distance between thecenter of the extract component peak envelope and the tracer peakenvelope (or other reference point) to the corresponding distancebetween the center of the raffinate component peak envelope and thetracer peak envelope. The rate of exchange of an extract component withthe desorbent can generally be characterized by the width of the peakenvelopes at half intensity. The narrower the peak width the faster thedesorption rate.

The pulse test run in Example I of the parent case using a Y typezeolite having Ca⁺⁺ ions at cation exchange sites showed the separationof psicose from carbohydrate mixtures containing the same. The calciumexchanged Y type zeolite of the example bound in an organic binder waspacked in a 8.4 mm diameter column having a total volume of 70 cc.

FIG. 1 provides a graphical representation of the adsorbent's relativestrength of the retention of the psicose, fructose and other sugars.

A consideration of the average midpoint for each concentration curvereveals a good separation of psicose from the other feed mixturecomponents. Psicose is clearly the most selectively, or strongly,retained component. Moreover, fructose is also selectively retained fromamong the remaining components, and may be separated from nonadsorbedcomponents and the more strongly adsorbed psicose by the Ca-Y adsorbentin the same or a different adsorbent bed. From the data obtained fromthis experiment the selectivities of Table 1 were calculated.

                  TABLE 1                                                         ______________________________________                                        Selectivity (B)                                                               ______________________________________                                        Psicose/Fructose  3.76                                                        Fructose/Mannose  1.68                                                        Fructose/Glucose  4.84                                                        ______________________________________                                    

These selectivities clearly establish the achievement of a high degreeof separation for psicose and also a satisfactory separate recovery offructose from the remaining components of the feed in a second extractin the first stage. According to the present invention, the psicose maybe further purified in a second stage by passing the first extract overthe same Ca-Y adsorbent column or another Ca-Y bed, e.g., elutioncolumn.

The examples shown below are intended to further illustrate the processof this invention and is not to be construed as unduly limiting thescope and spirit of said process.

EXAMPLE I

An aqueous feed, as described below in Table 2, was contacted, in acountercurrent flow, simulated moving bed system described above with acalcium-exchanged Y zeolite adsorbent, which was desorbed with water.The valve cycle was 1 hour, the temperature was 65° C. and the ratio ofadsorbent to feed, A/F, was 0.9. The feed was obtained by theisomerization of L-glucose at 37° C. and a pH of 10.6 for 48 hours.

                  TABLE 2                                                         ______________________________________                                        Feed         Wt. & Dry Solids                                                 ______________________________________                                        L-Glucose    51.0                                                             L-Fructose   28.7                                                             L-Mannose    7.7                                                              L-Psicose    5.3                                                              Unknown      3.4                                                              DP.sub.2     1.8                                                              DP.sub.3     1.4                                                              DP.sub.4 +   0.5                                                              ______________________________________                                    

In the first stage, most of the fructose was recovered in the extractstream and thus separated from the remaining feed components, as will beseen from the product analysis in Table 3 below, under conditions tooptimize fructose recovery free of psicose contamination. The extractfrom the first stage, consisting of highly pure L-fructose and a reducedconcentration of L-psicose was then treated in a column containingcalcium-exchanged Y faujasite. The L-psicose, being more stronglyadsorbed by the zeolite, was found greatly purified, in this finalextract stream while most of the L-fructose was obtained, substantiallyfree of L-psicose (0.5%), in the raffinate stream, with minor amounts ofother components, at a purity of 94.1%. L-psicose can be recovered instill greater concentration in the second extract by optimizing thefirst stage with respect to psicose recovery as will be shown in ExampleII. An analysis of the combined raffinate of two consecutive periods ofthe separation run show the composition of the raffinate to be high inL-fructose with small amounts of L-psicose and other components of thefeed mixture.

                  TABLE 3                                                         ______________________________________                                               Sorbex/w/Ca--Y                                                                             Ca--Y Elution Column                                             First Stage  Second Stage                                                       Extract        Final      Final                                      Component                                                                              (Feed to 2nd Stage)                                                                          Extract    Raffinate                                  ______________________________________                                        DP.sub.4 +                                                                              0.05          0.1        --                                         DP.sub.3 --             --         --                                         DP.sub.2 0.1            0.3        --                                         L-Frutose                                                                              91.8           83.2       94.1                                       L-Glucose                                                                              0.4            1.1        1.0                                        L-Mannose                                                                              2.8            4.6        2.2                                        L-Psicose                                                                              2.0            6.0        0.5                                        Others   2.8            4.6        2.2                                        ______________________________________                                    

EXAMPLE II

An aqueous feed, as described below in Table 4, was separated in acountercurrent flow, simulated moving bed system described above inwhich the adsorbent was a calcium-exchanged Y zeolite and the desorbentwas water. The valve cycle was 1 hour, the temperature was 65° C. andthe ratio of adsorbent to feed, A/F, was 1.15. The feed was obtained bythe isomerization of glucose at 37° C. and a pH of 10.6 for, 48 hours.

                  TABLE 4                                                         ______________________________________                                        Feed            Wt. % Dry Solids                                              ______________________________________                                        Glucose         55.4                                                          Fructose        29.2                                                          Mannose         6.1                                                           Psicose         5.6                                                           Others, including                                                                             3.7                                                           DP.sub.2, DP.sub.3, DP.sub.4 and                                              unknown                                                                       Total           100                                                           ______________________________________                                    

In the first stage, most of the fructose and psicose were separated fromthe remaining components in the extract stream, as will be seen from theproduct analysis in Table 5, below. The extract from the first stage,enriched in psicose may then be treated in an elution column containingcalcium-exchanged Y faujasite. The psicose, being more strongly adsorbedby the zeolite, will be greatly purified in the final extract stream,while most of fructose was obtained, substantially free of psicose(0.5%), in the raffinate stream, with minor amounts of other components,at purity of at least 90%. First extract composition data from fourfirst-stage runs (feed to the second stage separation), and thecomposition of the raffinate of Run No. 1 only are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        First Stage Separation                                                        First                                                                         Raffinate    First Extract                                                    Component                                                                             Run #1   Run #1   Run #2 Run #3 Run #4                                ______________________________________                                        L-Fructose                                                                            22.7     82.6     82.8   83.3   84.4                                  L-Glucose                                                                             66.8      .8      --     --     --                                    L-Mannose                                                                              6.7     2.9       2.7   3.3    3.8                                   L-Psicose                                                                              0.35    7.8      10.5   8.3    7.9                                   Other,  --       5.9       3.6   5.1    3.9                                   incl. DP.sub.2                                                                ______________________________________                                    

Using first stage extract recovery data from Example I (Example 111 ofthe parent case), recovery. of the components were calculated for asecond-stage separation of the first stage extracts of the above runs byelution in a Ca-Y zeolite packed column. The calculated results, tomaximize recovery of both fructose and psicose, are set forth in thefollowing table.

                  TABLE 6                                                         ______________________________________                                        Theoretical Distributions After 2nd Stage                                     (Ca--Y Elution Column)                                                        Component  Raffinate                                                                              Extract   Raffinate                                                                            Extract                                  ______________________________________                                                 Run #1       Run #2                                                  Glucose    0.2      2.1       --     --                                       Fructose   90.3     65.3      91.9   64.3                                     Mannose    2.4      4.1       2.2    3.7                                      Psicose    2.2      20.3      2.9    27.0                                     Other      4.9      8.3       2.9    5.0                                      Total      100.0    100.0     99.9   100.0                                             Run #3       Run #4                                                  Glucose    --       -1        --     --                                       Fructose   90.8     66.3      91.4   68.2                                     Mannose    2.7      4.6       3.2    5.3                                      Psicose    2.3      21.9      2.1    21.2                                     Other      4.2      7.2       3.3    5.3                                      Total      100.0    100.0     100.0  100.0                                    ______________________________________                                    

Further extrapolations can be made based on the theoretical separationspossible with the selectivities exhibited by the present adsorbent(Table 1) in order to obtain very. high purity psicose (withcorrespondingly low recoveries) by recovering in, or limiting, thesecond stage extract, to only that portion of the retention volume curverepresented in FIG. 1 greater than about 70 ml; psicose purity, up toabout 99% is possible with estimated recoveries of 10% to 20% Psicosepurities of 20 to 50% are prefered to obtain a sufficiently high percentrecovery for economic justification.

I claim:
 1. A process for recovering psicose having a purity of 20-99%(wt.) from a feed mixture comprising fructose and psicose and at leaston®other monosaccharide selected from the group consisting of glucoseand mannose which comprises contacting at adsorption conditions saidfeed mixture with a adsorbent comprising a type Y zeolite having calciumcations at exchangeable sites, selectivley adsorbing said psicosepreferentiallhy to said fructose and to the substantial exclusion of theother components, recovering an extract mixture comprising enrichedpsicose and fructose and then contacting said extract mixture with anadsorbent comprising a type Y zeolite having calcium cations atexchangeable sites, selectively adsorbing said psicose and recoveringhighly purified psicose by desorption at desorption conditions.
 2. Theprocess of claim 1 wherein said mixture comprises L-psicose, L-fructose,L-glucose and L-mannose.
 3. The process of claim 1 wherein saiddesorbent comprises water.
 4. The process of claim 1 wherein saidseparation is effected by means of a simulated moving bed flow scheme.5. The process of claim 4 wherein said simulated moving bed scheme usescountercurrent flow.
 6. The process of claim 1 wherein said simulatedmoving bed scheme uses cocurrent flow.
 7. The process of claim 1 whereina second extract, comprising fructose, is recovered from said step ofcontacting said feed mixture.